20th-century classical music describes orchestral works, chamber music, solo instrumental works (including keyboard music), electronic music, choral music, songs, operas, ballets, concertos, symphonies, and related forms, as well as fantasies, rhapsodies, fugues, passacaglias and chaconnes, variations, oratorios, cantatas, suites, improvisational and newly developed formal concepts such as variable and mobile forms, that have been written and performed since 1900. This era was without a dominant style and composers have created highly diverse kinds of music. Modernism, impressionism, post-romanticism, neoclassicism, expressionism, and, later, minimalism were all important movements. Atonality, serialism, musique concrète and electronic music were all developed during this period. Jazz was an important influence on many composers in this period.

At the turn of the century, music was characteristically late Romantic in style. Composers such as Gustav Mahler, Richard Strauss and Jean Sibelius were pushing the bounds of Post-RomanticSymphonic writing. At the same time, the Impressionist movement, spearheaded by Claude Debussy, was being developed in France. Debussy in fact loathed the term Impressionism: "I am trying to do 'something different—in a way realities—what the imbeciles call 'impressionism' is a term which is as poorly used as possible, particularly by art critics" (Politoske and Martin 1988, 419). Maurice Ravel's music, also often labelled as impressionist, explores music in many styles not always related to it (see the discussion on Neoclassicism, below).

Arnold Schoenberg, Los Angeles, 1948

Many composers reacted to the Post-Romantic and Impressionist styles and moved in quite different directions. The single most important moment in defining the course of music throughout the century was the widespread break with traditional tonality, effected in diverse ways by different composers in the first decade of the century. From this sprang an unprecedented "linguistic plurality" of styles, techniques, and expression (Morgan 1984, 458). In Vienna, Arnold Schoenberg developed atonality, out of the expressionism that arose in the early part of the 20th century. He later developed the twelve-tone technique which was developed further by his disciples Alban Berg and Anton Webern; later composers (including Pierre Boulez) developed it further still (Ross 2008, 194–96 and 363–64). Stravinsky (in his last works) explored twelve-tone technique, too, as did many other composers; indeed, even Scott Bradley used the technique in his scores for the Tom and Jerry cartoons (Ross 2008, 296).

From the early 1950s onwards, Cage introduced elements of chance into his music. Process music (Karlheinz StockhausenProzession, Aus den sieben Tagen; and Steve ReichPiano Phase, Clapping Music) explores a particular process which is essentially laid bare in the work.[vague] The term experimental music was coined by Cage to describe works that produce unpredictable results (Mauceri 1997, 197), according to the definition "an experimental action is one the outcome of which is not foreseen" (Cage 1961, 39). The term is also used to describe music within specific genres that pushes against their boundaries or definitions, or else whose approach is a hybrid of disparate styles, or incorporates unorthodox, new, distinctly unique ingredients.

At the end of the 19th century (often called the Fin de siècle), the Romantic style was starting to break apart, moving along various parallel courses, such as Impressionism and Post-romanticism. In the 20th century, the different styles that emerged from the music of the previous century influenced composers to follow new trends, sometimes as a reaction to that music, sometimes as an extension of it, and both trends co-existed well into the 20th century.[citation needed] The former trends, such as Expressionism are discussed later.

In the early part of the 20th century, many composers wrote music which was an extension of 19th-century Romantic music, and traditional instrumental groupings such as the orchestra and string quartet remained the most typical. Traditional forms such as the symphony and concerto remained in use. Gustav Mahler and Jean Sibelius are examples of composers who took the traditional symphonic forms and reworked them. (See Romantic music.) Some writers hold that the Schoenberg's work is squarely within the late-Romantic tradition of Wagner and Brahms (Neighbour 2001, 582) and, more generally, that "the composer who most directly and completely connects late Wagner and the 20th century is Arnold Schoenberg" (Salzman 1988, 10).

Impressionism started in France as a reaction, led by Claude Debussy, against the emotional exuberance and epic themes of German Romanticism exemplified by Wagner. In Debussy's view, art was a sensuous experience, rather than an intellectual or ethical one. He urged his countrymen to rediscover the French masters of the 18th century, for whom music was meant to charm, to entertain, and to serve as a "fantasy of the senses" (Machlis 1979, 86–87).

At its conception, Futurism was an Italian artistic movement founded in 1909 by Filippo Tommaso Marinetti; it was quickly embraced by the Russian avant garde. In 1913, the painter Luigi Russolo published a manifesto, L'arte dei rumori (The Art of Noises), calling for the incorporation of noises of every kind into music (Russolo 1913). In addition to Russolo, composers directly associated with this movement include the Italians Silvio Mix, Nuccio Fiorda, Franco Casavola, and Pannigi (whose 1922 Ballo meccanico included two motorcycles), and the Russians Artur Lourié, Mikhail Matyushin, and Nikolai Roslavets.

Postmodernism is a reaction to modernism, but it can also be viewed as a response to a deep-seated shift in societal attitude. According to this latter view, postmodernism began when historic (as opposed to personal) optimism turned to pessimism, at the latest by 1930 (Meyer 1994, 331).

John Cage is a prominent figure in 20th-century music, claimed with some justice both for modernism and postmodernism because the complex intersections between modernism and postmodernism are not reducible to simple schemata (Williams 2002, 241). His influence steadily grew during his lifetime. He often uses elements of chance: Imaginary Landscape No. 4 for 12 radio receivers, and Music of Changes for piano. Sonatas and Interludes (1946–48) is composed for a prepared piano: a normal piano whose timbre is dramatically altered by carefully placing various objects inside the piano in contact with the strings. Currently Postmodernism includes composers who react against the Avant-Garde and experimental styles of the late 20th century such as Astor Piazzolla,Argentina and Miguel del Aguila, USA

In the later 20th century, composers such as La Monte Young, Arvo Pärt, Philip Glass, Terry Riley, Steve Reich, and John Adams began to explore what is now called minimalism, in which the work is stripped down to its most fundamental features; the music often features repetition and iteration. An early example is Terry Riley's In C (1964), an aleatoric work in which short phrases are chosen by the musicians from a set list and played an arbitrary number of times, while the note C is repeated in eighth notes (quavers) behind them. Steve Reich's works Piano Phase (1967, for two pianos), and Drumming (1970–71, for percussion, female voices and piccolo) employ the technique called phasing in which a phrase played by one player maintaining a constant pace is played simultaneously by another but at a slightly quicker pace. This causes the players to go "out of phase" with each other and the performance may continue until they come back in phase.

Philip Glass's 1 + 1 (1968) employs the additive process in which short phrases are slowly expanded. La Monte Young's Compositions 1960 employs very long tones, exceptionally high volumes and extra-musical techniques such as "draw a straight line and follow it" or "build a fire". Michael Nyman argues that minimalism was a reaction to and made possible by both serialism and indeterminism (Nyman 1999, 139). (See also experimental music.)

The development of recording technology made all sounds available for potential use as musical material. Electronic music generally refers to a repertory of art music developed in the 1950s in Europe, Japan, and the Americas. The increasing availability of magnetic tape in this decade provided composers with a medium which allowed recording sounds and then manipulating them in various ways. All electronic music depends on transmission via loudspeakers, but there are two broad types: acousmatic music, which exists only in recorded form meant for loudspeaker listening, and live electronic music, in which electronic apparatus are used to generate, transform, or trigger sounds during performance by musicians using voices, traditional instruments, electro-acoustic instruments, or other devices. Beginning in 1957, computers became increasingly important in this field (Emmerson and Smalley 2001). When the source material was acoustical sounds from the everyday world, the term musique concrète was used; when the sounds were produced by electronic generators, it was designated electronic music. After the 1950s, the term "electronic music" came to be used for both types. Sometimes such electronic music was combined with more conventional instruments, Stockhausen's Hymnen, Edgard Varèse's Déserts, and Mario Davidovsky's series of Synchronisms are three examples.

Various prominent composers from the 20th century are not associated with any widely recognised compositional movement. The list below includes some of them, as well as various significant classifiable composers who are not mentioned in the preceding parts of this article:

Emmerson, Simon, and Denis Smalley. 2001. "Electro-Acoustic Music". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.

Evans, Peter. 1979. The Music of Benjamin Britten. London: Dent.

Fanning, David. 2001. "Expressionism". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.

Neighbour, O. W. 2001. "Schoenberg, Arnold". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell, 22:577–604. London: Macmillan Publishers.

Nyman, Michael. 1999. Experimental Music: Cage and Beyond, second edition. Music in the Twentieth Century. Cambridge and New York: Cambridge University Press. ISBN 0-521-65383-5.

Pasler, Jann. 2001a. "Impressionism". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers; New York: Grove's Dictionaries of Music.

Pasler, Jann. 2001b. "Neo-romantic". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.